The background in related art is described by Vaage et al. in U.S. Pat. No. 7,684,281. In seismic exploration, geophysical data are obtained by applying acoustic energy to the earth from an acoustic source and detecting seismic energy reflected from interfaces between different layers in subsurface formations. The seismic wavefield is reflected when there is a difference in acoustic impedance between the layers on either side of the interface. Typically in marine seismic exploration, a seismic streamer is towed behind an exploration vessel at a water depth normally between about six to about nine meters, but can be towed shallower or deeper. Hydrophones are included in the streamer cable for detecting seismic signals. A hydrophone is a submersible pressure sensor that converts pressure waves into electrical or optical signals that are typically recorded for signal processing, and evaluated to estimate characteristics of the subsurface of the earth.
In a typical geophysical exploration configuration, a plurality of streamer cables are towed behind a vessel. One or more seismic sources are also normally towed behind the vessel. The seismic source, which is often an air gun array, but may also be a water gun array or other type of source known to those of skill in the seismic art, transmits seismic energy or waves into the earth and the waves are reflected back by interfaces in the earth and recorded by sensors in the streamers. Winged hydrodynamic actuators are typically employed to maintain the cables in the desired lateral position while being towed. Alternatively, the seismic cables are maintained at a substantially stationary position in a body of water, either floating at a selected depth or lying on the bottom of the body of water, in which case the source may be towed behind a vessel to generate acoustic energy at varying locations, or the source may also be maintained in a stationary position.
When the reflected wave reaches the streamer cable, the wave is detected by the hydrophones in the streamer cable as the primary signal. The reflected wave also continues to propagate to the water/air interface at the water surface, from which the wave is reflected downwardly, and is again detected by the hydrophones in the streamer cable. The water surface is a good reflector and the reflection coefficient at the water surface is nearly unity in magnitude and is negative in sign for pressure signals. The waves reflected at the surface will thus be phase-shifted 180° relative to the upwardly propagating waves. The downwardly propagating wave recorded by the receivers is commonly referred to as the surface reflection or the “ghost” signal. Because of the surface reflection, the water surface acts like a filter, which creates spectral notches in the recorded signal, making it difficult to record data outside a selected bandwidth. Because of the influence of the surface reflection, some frequencies in the recorded signal are amplified (constructive interference) and some frequencies are attenuated (destructive interference).
Maximum attenuation will occur at frequencies for which the propagation distance between the detecting hydrophone and the water surface is an integer multiple of one-half wavelength. Maximum amplification will occur at frequencies for which the propagation distance between the detecting hydrophone and the water surface is an integer multiple of one-quarter wavelength. The wavelength of the acoustic wave is equal to the velocity divided by the frequency, and the velocity of an acoustic wave in water is about 1500 meters/second. Accordingly, the location in the frequency spectrum of the resulting first (lowest-frequency) spectral notch can be readily determined. For example, for a seismic streamer at a depth of 7 meters, and waves with vertical incidence, maximum attenuation will occur at a frequency of about 107 Hz and maximum amplification will occur at a frequency of about 54 Hz.
It has not been common practice to tow streamer cables deeper than about nine meters because the location of the lowest-frequency spectral notch in the frequency spectrum of the signal detected by a hydrophone substantially diminishes the utility of the recorded data. It has also not been common practice to tow streamer cables at depth less than six meters, because of the significant increase in surface related noise induced in the seismic streamer data.
It is also common to perform marine seismic operations in which sensors are deployed at the water bottom. Such operations are typically referred to as “ocean bottom seismic” operation seismic operations, both pressure sensors (hydrophones) and particle motion sensors (geophones, accelerometers) are deployed at the ocean floor to record seismic data.
A particle motion sensor, such as a geophone, has directional sensitivity, whereas a pressure sensor, such as hydrophone, does not. Accordingly, the upgoing wavefield signals detected by a geophone and hydrophone located close together will be in phase, while the downgoing wavefield signals will be recorded 180° out of phase if the geophone is oriented in a particular direction. Various techniques have been proposed for using this phase difference to reduce the spectral notches caused by the surface reflection and, if the recordings are made on the seafloor, to attenuate water borne multiples. It should be noted that an alternative to having the geophone and hydrophone co-located, is to have sufficient spatial density of sensors so that the respective wavefields recorded by the hydrophone and the geophone can be reconstructed (interpolated) at a convenient location in the vicinity of the spatial distribution of sensors.
U.S. Pat. No. 4,486,865 to Ruehle, for example, teaches a system for suppressing ghost reflections by combining the outputs of pressure and velocity detectors. The detectors are paired, one pressure detector and one velocity detector in each pair. A filter is said to change the frequency content of at least one of the detectors so that the ghost reflections cancel when the outputs are combined.
U.S. Pat. No. 5,621,700 to Moldovenu also teaches using at least one sensor pair comprising a pressure sensor and a velocity sensor in an ocean bottom cable in a method for attenuating ghosts and water layer reverberations.
U.S. Pat. No. 4,935,903 to Sanders et al, teaches a marine seismic reflection prospecting system that detects seismic waves traveling in water by pressure sensor-particle velocity sensor pairs (e.g. hydrophone-geophone pairs) or alternately, vertically-spaced pressure sensors. Instead of filtering to eliminate ghost reflection data, the system calls for enhancing primary reflection data for use in pre-stack processing by adding ghost data.
U.S. Pat. No. 4,979,150 to Barr provides a method for marine seismic prospecting said to attenuate coherent noise resulting from water column reverberation by applying a scale factor to the output of a pressure transducer and a particle velocity transducer positioned substantially adjacent to one another in the water. Barr states that the transducers may be positioned either on the ocean bottom or at a location in the water above the bottom, although the ocean bottom is said to be preferred.
U.S. Pat. No. 7,239,577, to Tenghamn describes a particle motion sensor for use in a streamer cable and a method for equalizing and combining the output signals of the particle motion sensor and a co-located pressure gradient sensor.
As the cited patents show, it is well known in the art that pressure and particle motion signals can be combined to derive both the up-going and the down-going wavefield. For sea floor recordings, the up-going and down-going wavefields may subsequently be combined to remove the effect of the surface reflection and to attenuate water borne multiples in the seismic signal. For towed streamer applications, however, the particle motion signal has been regarded as having limited utility because of the high noise level in the particle motion signal. However, if particle motion signals could be provided for towed streamer acquisition, the effect of the surface reflection could be removed from the data.
U.S. Pat. No. 7,123,543 describes a procedure for attenuating multiples by combining up- and down-going wavefields, measured in the water column, where the wavefields are calculated from combining pressure sensors like hydrophones and motion sensors like geophones. The procedure assumes, however, that both the pressure and the motion data have the same bandwidth.
It has been difficult to achieve the same bandwidth in the motion sensor data as in the pressure sensor data, however, because of the noise induced by vibrations in the streamer, which is sensed by the particle motion sensors. The noise is, however, mainly confined to lower frequencies. One way to reduce the noise is to have several sensors in series or in parallel. This approach, however, does not always reduce the noise enough to yield a signal-to-noise ratio satisfactory for further seismic processing.
A combination of acoustic pressure and particle velocity can in principle be used to discriminate the direction of acoustic wavefront. This technique has a long history in the world of ‘velocity’ microphones.
In the field of marine geophysics, acoustic particle velocity sensing is often done with geophones (typically electrodynamic velocity sensors). The motion of a neutrally-buoyant cable is taken to be a good analog of the acoustic particle velocity, at least over some frequency range and some angle of incidence range. To minimize cost and complexity some vendors use a single axis gimbaled sensor on the assumption that only vertically-oriented wavefronts are of primary interest.
Historically, vertically oriented wavefronts were in fact the primary concern, but in modern geophysics there is increased interest in wavefronts arriving from a broad range of angles, so the gimbaled single axis sensor is not optimal.
High quality gimbals are not inexpensive, and even the best introduce the possibility of slip ring noise, and it is common practice to add fluid damping so that the geophone sensor orientation may lag the actual cable orientation in the presence of roll. Such a lag would introduce errors in the measured acoustic particle velocity.
In the case of ‘solid’ cable streamers such as Sercel's Sentinel® streamer, gimbals pose a very difficult problem in that the gimbaled sensor needs to have its center of gravity exactly on the cable center of gravity, yet that space is occupied by stress member and electrical wiring. SENTINEL® is a registered trademark of Sercel, Inc. A pair of orthogonal particle motion sensors with their active axes passing through the cable axis bypass the geometrical problems as well as the potential for lag and slip ring noise while also allowing for the possibility of discrimination of other-than-vertical wavefront arrivals.
For orthogonal particle motion sensors, separate tilt (rotation) sensing means must be provided (e.g. accelerometers with adequate DC accuracy) in order to determine direction based on gravity. A pair of orthogonal high quality DC-responsive accelerometers could serve both for velocity sensing and directional sensing, but the embodiments described herein use less expensive components.
Every sensor poses a cost in terms of data acquisition bandwidth. Obviously the single gimbaled velocity sensor is a lowest-cost approach, but with local signal processing the dual axis particle motion sensor plus tilt sensing can be reduced to an equivalent bandwidth load if the functionality of off-vertical discrimination is sacrificed.
In conclusion, the dual orthogonal sensor approach resolves difficult design problems as well as providing much more valuable information to the geophysicist in the way of velocity components.
Thus, a need exists for a method for obtaining a useful particle motion signal with a satisfactory signal-to-noise ratio at low frequencies. In particular, a need exists for a method to generate a particle motion signal with substantially the same bandwidth as a recorded pressure signal, for particle motion and pressure sensors located in a towed marine seismic streamer. Unfortunately, the proposed solutions thus far described are far too complex and expensive to find wide application in the field, and the complexity of these solutions leads to unacceptably high failure rates in operation. In particular there exists a need for a simple, inexpensive structure to combine pressure and particle motion signals in a marine seismic cable to eliminate or minimize ghosts. The invention disclosed herein is directed to fulfilling that need in the art.